Liquid crystal displays (LCDs) are passive displays which depend upon external sources of light for illumination. Most commonly, LCDs are manufactured in an active matrix addressed format in which an array of diodes, metal-insulator-metal (MIM) devices, or thin film transistors (TFTs) supplies an electronic switch to each pixel. Two sheets of glass form the structure of the display. The separation of the sheets is the critical gap dimension of 5–10 um that contains the liquid crystal material. In order to maintain uniformity of the gap dimension, extremely precise flatness of the glass sheet is required.
Active matrix liquid crystal displays (AMLCDs) employ an active device such as a diode or thin film transistor at each pixel thereby enabling high contrast and high response speed. Although many display devices currently utilize amorphous silicon (a-Si), the processing of which may be accomplished at temperatures under 450° C., polycrystalline-silicon (poly-Si) processing is preferred. Poly-Si has a much higher drive current and electron mobility thereby increasing the response time of the pixels. Further, it is possible, using poly-Si processing, to build the display drive circuitry directly on the glass substrate. By contrast, a-Si requires discrete driver chips that must be attached to the display periphery utilizing integrated circuit packaging techniques. Poly-Si processing methods operate at higher temperatures than those employed with a-Si TFTs. Such processes enable formation of poly-Si films having extremely high electron mobility (for rapid switching) and excellent TFT uniformity across large areas. The actual temperature required is mandated by the particular process utilized in fabricating the TFTs. Those TFTs with deposited gate dielectrics require 600–650° C., while those with thermal oxides require about 800° C. Both a-Si and poly-Si processes demand precise alignment of successive photolithographic patterns, thereby necessitating that the thermal shrinkage of the substrate be kept low.
The temperature requirements have mandated the use of glasses exhibiting high strain points in order to avoid thermal deformation at temperatures above 600° C.
It is generally accepted that four properties are deemed mandatory for a glass to exhibit in order to fully satisfy the needs of a substrate for LCDs:
First, the glass must be essentially free of intentionally added alkali metal oxide to avoid the possibility that alkali metal from the substrate can migrate into the transistor matrix;
Second, the glass substrate must be sufficiently chemically durable to withstand the reagents used in the TFT deposition process;
Third, the expansion mismatch between the glass and the silicon present in the TFT array must be maintained at a relatively low level even as processing temperatures for the substrates increase; and,
Fourth, the glass must be capable of being produced in high quality thin sheet form at low cost; that is, it must not require extensive grinding and polishing to secure the necessary surface finish.
The last requirement is a particularly difficult one to achieve inasmuch as it demands a sheet glass production process capable of producing essentially finished glass sheet, such as the overflow downdraw sheet manufacturing process described in U.S. Pat. No. 3,682,609. That process requires a glass exhibiting a very high viscosity at the liquidus temperature plus long term stability, e.g. periods of 30 days, against devitrification at melting and forming temperatures.
Most glasses to date that fulfill the requirements set forth above are based on eutectic compositions in the alkaline earth boroaluminosilicate systems. The present invention explores a compositional area whose benefits for use as a substrate for display devices will be made evident.